The present invention relates to electronic circuits and, more particularly, to a circuit, system, structure and method for protecting various circuits from unwanted reverse current flow under various conditions.
Electronic circuits are employed in numerous consumer and industrial applications. Such applications are extremely diverse; from computers, to wireless portable communications devices, to industrial controls, etc. In such myriad applications, the electronic circuits, typically fabricated and packaged as integrated circuit (IC) chips, are utilized to achieve various types of functions, for example, digital control, digital-to-analog (D/A) or analog-to-digital (A/D) conversion, mixed signal analysis, etc.
One exemplary circuit application relates to the drive and control of DC cooling fans which are employed in systems such as portable, notebook computers. Such cooling fans operate to cool the various notebook computer components located therein and are controlled using, for example, a DC brushless motor. The DC brushless motor, in turn, is driven and controlled by a control system, as illustrated in prior art FIG. 1, and designated at reference numeral 10.
The exemplary system 10 includes a fan speed controller circuit 12 and a fan driver circuit 14. The fan speed controller circuit 12 provides a control signal, for example, a pulse width modulation (PWM) signal to the fan driver circuit 14 based on a desired fan speed. The system 10 further may include a motor position detection circuit 16 such as a Hall sensor which monitors the relative rotor/stator position of the DC brushless motor 18 and provides motor position information back to the fan driver circuit 14 for commutation control thereof.
The system 10 of prior art FIG. 1 is powered by a system power supply 20, for example a 5V DC battery. In such systems where circuits such as the fan driver circuit 14 interface with the power supply 20, protection mechanisms such as an external blocking diode 22 often are employed to prevent a reverse bias system condition from causing a reverse current to flow through the fan driver circuit 14 back to the power supply 20. In some cases such reverse current flow can be destructive to the fan driver circuit 14, the system power supply 20 or the fan itself. In reverse bias conditions, for example, when the fan is plugged in backwards, a reverse current will conduct through the circuit 14 to the power supply 20. By employing the blocking diode 22 as illustrated in prior art FIG. 1, the diode becomes reverse biased under such conditions and no such reverse current conducts therethrough.
Use of blocking diodes such as the diode 22 of FIG. 1, however, has several disadvantages. For example, such a solution limits the power supply voltage of circuits such as the fan driver circuit 14 in its normal operating mode. For example, the fan driver circuit 14 may have a minimum voltage supply parameter of about 3.3V and the forward biased diode drop is relatively high (e.g., about 0.5-0.7V) under normal operating conditions. Furthermore, due to the reduced supply voltage provided to the fan driver circuit 14, the maximum revolutions-per-minute (RPMS) of the fan driven by the motor 18 is reduced. That is, since the RPM of the fan is a function of the PWM and the current supplied to the motor 18, a reduced operating voltage reduces the current for any given PWM signal, thereby reducing the RPM of the fan. In addition, because the blocking diode 22 is typically an external type device, the component negatively limits an amount of miniaturization of a system printed circuit board (PCB) and/or makes such a PCB layout more complex, increases PCB component count and increases cost associated therewith.
There is a need in the art for improvements in reverse bias protection devices, circuits or structures.
The present invention relates to a system and method of preventing reverse currents in circuit applications without sacrificing appreciable voltage headroom under normal operating conditions.
According to one aspect of the present invention, a reverse bias protection structure is disclosed. The protection structure comprises a PMOS transistor and a Schottky diode structure which are configured so as to conduct through the PMOS transistor under forward bias conditions and block current via the Schottky diode structure under reverse bias conditions. Because the PMOS transistor on-resistance is low, its forward bias voltage drop is substantially less than prior art blocking diode type devices, and thus provides additional voltage headroom to circuits which the protection structure is designed to protect.
According to another aspect of the present invention, a system for controlling a component receiving electrical energy is disclosed. The system comprises a system power supply, a component control circuit, the component to be controlled, and a protection structure. The protection structure is associated with the component control circuit and is operable to prevent reverse currents therethrough to the power supply under reverse bias conditions and substantially pass the power supply voltage to the component control circuit under forward bias conditions, wherein the voltage drop across the protection structure in forward bias conditions is less than a forward bias diode drop.
According to yet another aspect of the present invention, a protection structure is disclosed which comprises a PMOS transistor having a source, a gate, a drain and a backgate associated therewith. The protection structure further comprises a Schottky diode structure having an anode and a cathode. The anode of the Schottky diode is coupled to the source of the PMOS transistor and the cathode is coupled to the backgate. The source of the PMOS transistor forms a protection structure input and the drain forms a protection structure output, respectively.
In use, the protection structure may have the input coupled to a power supply or power supply voltage rail and the output coupled to a circuit wishing to employ energy associated with the power supply or rail. In forward bias, normal operating conditions, the gate terminal of the PMOS is coupled to a low voltage potential such that the PMOS transistor is on and the power supply voltage is passed substantially to the output with the voltage drop thereacross being about a VDS(ON) of the PMOS transistor, which is substantially less than a forward bias diode voltage drop. In reverse bias conditions, the source terminal and anode of the Schottky diode are coupled to a low voltage potential and the gate is coupled to a higher potential such that the PMOS transistor is turned off. The backgate potential of the PMOS transistor and the low voltage of the anode of the Schottky diode under such conditions causes the Schottky structure to be reverse biased, and thus prevent reverse current flow through the structure under reverse bias conditions.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such embodiments and their equivalents. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.